A cellulose acylate film is used in various photographic or optical elements because it is tough and has enough flame retardant properties. In fact, the cellulose acylate film is a representative photographic support. Since having an optical isotropy, the cellulose acylate film is also used in a liquid crystal display device, which has recently extended its market. Typically, the cellulose acylate film is used as a protective film of a polarizing plate or a color filter in the liquid crystal display device.
The cellulose acylate film is generally produced according to a solvent cast method or a melt cast method. The solvent cast method comprises the steps of: casting a solution of cellulose acylate dissolved in a solvent (dope) onto a metal support, and evaporating the solvent to form a film. The melt cast method comprises the steps of: heating cellulose acylate to melt, casting the melted cellulose acylate on a metal support, and cooling it to form a film. The solvent cast method gives a film better in planeness than the melt cast method, and hence is usually adopted in practice. There are many publications describing the solvent cast method. With respect to the solvent cast method, it has been recently studied to shorten the time from the step of casting the dope onto the metal support to the step of peeling the formed film from the support, and thereby to improve the productivity of film forming. For example, Japanese Patent Publication No. 5(1993)-17844 describes a process in which such a concentrated dope is cast on a cooling drum that the time from casting to peeling is shortened.
The solvent used in the solvent cast method is required to not only dissolve cellulose acylate but also satisfy various conditions. The viscosity and the polymer concentration of the solution (dope) should be appropriately adjusted to form a flat plane film having uniform thickness. The dope also should have enough stability. Further, the dope should be easily set to gel. Furthermore, the formed film should be easily peeled from the support. In order to satisfy these requirements, the most appropriate solvent must be selected. Moreover, the solvent should be evaporated easily enough to remain little in the film. Various organic solvents have been proposed as the solvent of cellulose acylate. However, only methylene chloride can satisfy all the above-mentioned requirements. Accordingly, solvents other than methylene chloride have not been practically used.
However, the use of hydrocarbon halides such as methylene chloride has been recently restricted severely to protect the global environmental conditions. Further, having a low boiling point (41° C.), methylene chloride is apt to vaporize in the production process of the film. Accordingly, methylene chloride can cause problems in the working environment. The process is, therefore, conducted in a closed system. However, there is a technical limitation on sealing methylene chloride in the closed system. In consideration of this, it is urgently necessary to search a cellulose acylate-dissolving solvent substituting for methylene chloride.
By the way, acetone, which is a widely used organic solvent, has a relatively low boiling point (56° C.), and hence the process of evaporating acetone does not need large thermal energy. Further, acetone has few problems on the human body and the global environmental conditions, as compared with the chlorinated organic solvents. However, cellulose acylate has a poor solubility in acetone. Cellulose acetate having a substitution degree of 2.70 (acetic acid content: 58.8%) or less can be slightly dissolved in acetone, but the solubility in acetone is further lowered if the substitution degree is more than 2.70. Cellulose acetate having a substitution degree of 2.80 (acetic acid content: 60.1%) or more can be swelled but not dissolved in acetone.
J. M. G. Cowie et al. report in Makromol., Chem., vol. 143 (1971), pp.105, that cellulose acylate having a substitution degree in the range of 2.80 to 2.90 is dissolved in acetone through a specific process. The process comprises the steps of cooling the cellulose acylate (in the reported process the acyl group is restricted to acetyl group) in acetone at a temperature of −80 to −70° C., and warming it to obtain 0.5 to 5 wt. % solution of cellulose acylate in acetone. This method, in which the mixture of cellulose acylate is cooled in an organic solvent to obtain a solution, is hereinafter referred to as “cooling dissolution method”. The solution of cellulose acylate in acetone is also reported by K. Kamide et al., Bulletin of Textile Machinery Society, Vol. 34, pp. 57 (1981). The report (written in Japanese) is entitled “Dry spinning process using acetone solution of triacetyl cellulose”. In the report, the cooling dissolution method is applied to the art of fiber spinning. The experiments described in the report examine the mechanical strength, the dyeing property and the sectioned shape of the fiber obtained by the cooling dissolution method. In the report, 10 to 25 wt. % solutions of cellulose acetate are used to form a fiber.
With respect to the solvent and the dissolution method of cellulose acylate, the inventors have been studied various matters such as chlorinated and non-chlorinated solvent components of the dope, preparation of the dope and casting process for film formation. As a result, it was confirmed that the productivity of the film greatly depends upon how easily the formed film is peeled from the metal support and how easily the film is conveyed during its being dried, in the case where the film is produced by casting the cellulose acylate solution. In other words, it is important that the film is easily peeled off and that the peeled wet film is rigid enough not to deform in conveying.